First-order photometric calibration for 2MASS is evaluated nightly using observations of calibration fields made at regular intervals. Photometry of standard stars in these fields is used to derive the photometric zero points in each of the three survey bandpasses as a function of time during each night. Atmospheric extinction coefficients are derived from 2MASS observations made over long periods.
Calibration Tiles and Observations
2MASS calibration tiles are 1° long in declination (plus overhead), and are covered by scans containing 48 frames. Each calibration observation consists of six independent scans of a calibration tile, made in the same freeze-frame scanning mode and scan rate as the normal survey tiles. Each scan is made in alternating directions and is cross-stepped 5" in RA from the previous one to minimize systematic pixel effects.
At the beginning of the Survey, two calibration fields were observed every two hours during a night. Beginning on 11 October 1997 UT, the calibration strategy was modified so that one calibration field was observed approximately every hour during a night. Normal 2MASS operations are started with a calibration observation, and the actual calibration interval is adjusted so that the final calibration observation is coincident with morning twilight.
The calibration strategy emphasizes the measurement of the photometric zero point of the night, so a few calibration fields are and measured multiple times during a night. The selected fields are alternated so the same field is rarely observed on sequential hours. Repeated measurements of calibration fields during at night at a variety of elevation angles are used to develop long-term atmospheric extinction statistics.
2MASS Calibration Fields
The 2MASS calibration fields, or tiles, were selected to be centered on one or more primary calibration stars drawn from either the list of faint near infrared standard stars developed by Persson et al. (1998 AJ, 116, 2475) or the UKIRT group of faint, equatorial near infrared standard stars (Casali and Hawarden 1992, JCMT-UKIRT Newsletter, No. 4, 33)
Calibrators were selected so that there would be a set of equatorial and ±30° declination fields on approximately 2h RA centers around the sky, if possible. The equatorial fields can be observed from both hemispheres to develop short-term tie points between the observatories, and the high declination fields will transit close to the zenith at Mt. Hopkins or Cerro Tololo, providing low airmass calibration. Because the Persson et al. and UKIRT lists have very few stars at +30° declination in the 20-22h range, a field was defined in that area and the standards in it were calibrated internally to 2MASS over the first few months of observations. Incidentally, this field was selected to cover the Abell 2409 galaxy cluster so that long-term monitoring of galaxy photometric performance in the Survey could be made. A listing of all of the northern 2MASS calibration fields, and the primary calibration stars in them is given in Table 1.
Secondary Calibration Stars
Although each 2MASS calibration tile is centered on one primary calibration star, there are dozens if not hundreds of high signal-to-noise stars measured during every scan of those tiles. It did not take long to begin to accumulate a wealth of highly accurate relative photometry for the secondary stars in each field, calibrated in the internal 2MASS system. Within a few months of the start of survey observations, the secondary standard star photometry was included into the calibration calculations, greatly improving the accuracy of the zero point determinations for each night. The 2MASS secondary star network will continue to improve and grow as the survey progresses, and these will be incorporated into the photometric solutions. Table 1 includes a listing of the calibrated magnitudes and positions for all northern secondary calibration stars.
[Last Update: 1999 May 4, by R. Cutri, S. Wheelock, E. Howard]